This invention relates to an optoelectronic integrated circuit (OEIC) having a semiconductor laser, light emitting diodes and other light emitting devices, and heterojunction bipolar transistors (HBTs) and other electronic devices for driving these light emitting devices integrated on a same substrate.
The optoelectronic integrated circuit having a semiconductor laser, light emitting diodes and other light emitting devices, and heterojunction bipolar transistors and other electronic devices for driving these light emitting devices integrated on a same substrate is expected to be high in performance, high in reliability and low in price as the light source to be used in long distance optical fiber communication using local area network (LAN) and silica glass fiber, and high speed data communication by optical fibers between computers. Various methods have been proposed for the OEIC. For example, as disclosed in Applied Physics Letters, pp. 191-193, vol. 45 (published 1984), a conventional device integrating a buried heterostructure (BH) laser and a heterojunction bipolar transistor is known. The method of fabrication and structure of this prior art are described below. As shown in FIG. 4A, in order to form a double heterojunction (DH) on an N type InP substrate 301, an N type InP cladding layer 302 (carrier density 5.times.10.sup.17 cm.sup.-3, thickness 5 .mu.m), an InGaAsP active layer 303 (band gap wavelength .lambda.g=1.3 .mu.m, thickness 0.15 .mu.m), and a P type InP cladding layer 304 (carrier density 5.times.10.sup.17 cm.sup.-3, thickness 2.5 .mu.m), and a P type InGaAsP contact layer 305 (band gap wavelength .lambda.g=1.3 .mu., thickness 0.5 .mu.m) are sequentially formed by liquid phase epitaxy (LPE) technique. A silicon dioxide film (SiO.sub.2 film) 306 formed in a thickness of 50 nm by a thermal chemical vapor deposition (CVD) method is formed in stripe in a width of, for example, 7 .mu.m, and using this silicon dioxide film 306 as the mask, a dove-tail shaped mesa 307 is formed as shown in FIG. 4B with the bromine diluted to 2 wt. % by methanol. Afterwards, by the liquid phase epitaxial method, the dove-tail shaped mesa 307 is sequentially (buried up) with a P type InP separation layer 311 (carrier density 1.times.10.sup.17 cm.sup.-3, thickness 1 .mu.m), an N type InP collector layer 312 (carrier density 1.times.10.sup.17 cm.sup.-3, thickness 2 .mu.m), a P type InGaAsP base layer 313 (band gap wavelength .lambda.g=1.1 .mu.m, carrier density 1.times.10.sup.17 cm.sup.-3, thickness 0.3 .mu.m), and an N type InP emitter layer 314 (carrier density 5.times.10.sup.17 cm.sup.-3, thickness 0.7 .mu.m), an N type InGaAsP emitter contact layer 315 (band gap wavelength .lambda.g=1.3 .mu.m, carrier density 1.times.10.sup.17 cm.sup.-3, thickness 0.5 .mu.m) (FIG. 4C).
In the next step, a 200 nm thick silicon nitride film (Si.sub.3 N.sub.4 film) is deposited by plasma-assisted CVD method on the surface of laser part 316 and emitter contact layer 315 in the heterojunction bipolar transistor part 317, and using it as the mask, the emitter contact layer 315 is selectively etched by a mixed solution of H.sub.2 SO.sub.4 :H.sub.2 O.sub.2 :H.sub.2 O=1:1:5. Next, using a newly deposited 300 nm thick silicon nitride film 318 as the mask, the emitter layer 314 and base layer 313 are selectively etched by a mixed solution of HCl:H.sub.3 PO.sub.4 =1:2 and a mixed solution of H.sub.2 SO.sub.4 :H.sub.2 O.sub.2 :H.sub.2 O=1:1:5 respectively, and a mesa of the laser part 316 and the transistor part 317 is formed, so that the shape as shown in FIG. 4D is obtained.
Furthermore, using the newly deposited 300 nm thick silicon nitride film 319 and 200 nm thick silicon dioxide film 320 as the mask by opening holes in these two films, a P type impurity Zn is diffused for 1 hour at 500.degree. C. until reaching a P type InP separation layer 311 in a fused silica ampule using ZnP.sub.2 as diffusion source, and a isolation region 321 for electric isolation between heterojunction bipolar transistors is formed. Further opening holes in the graft base of the two films 319, 320, a P type impurity Zn is diffused for 10 minutes at 500.degree. C. until reaching or passing through the base layer 313 in a fused silica ampule using ZnP.sub.2 as diffusion source, thereby forming a graft base region 322 as shown in FIG. 4E.
After completely removing the two films 319, 320, a 400 nm thick silicon dioxide film 323 is deposited, and 10/15/300 nm thick Au/Sn/Au is evaporated, and an emitter electrode 324 and a collector electrode 325 are formed by the lift-off method. Furthermore, evaporating 10/10/300 nm thick Au/Zn/Au, an anode electrode 326, a base electrode 327, and a electrode 328 of isolation region are formed by the lift-off method as shown in FIG. 4F.
Then, in order to electrically separate the laser part 316 and heterojunction bipolar transistor part 317, an opening is made in the silicon dioxide film 323, and the collector layer 312 and separation layer 311 are etched by using a mixed solution of HCl:H.sub.3 PO.sub.4 =1:2, and a separation groove is formed, and an polyimide film 329 is buried in the separation groove as shown in FIG. 4G.
Evaporating 100/1000 nm thick Ti/Au, an interconnection 330 between devices is formed by the lift-off method. When the back side of the N type InP substrate 301 is polished to a thickness of 100 .mu.m and a cathode electrode 331 is formed by evaporating 10/15/300 nm thick Au/Sn/Au, an optoelectronic integrated circuit having BH laser and heterojunction bipolar teansistor integrated together is obtained as shown in FIG. 4H.
FIG. 5 shows the equivalent circuit of the conventional optoelectronic integrated circuit of FIG. 4. In this optoelectronic integrated circuit, three heterojunction bipolar transistors are integrated, and they compose a differential switch for driving the BH laser, and since the heterojunction bipolar transistors are provided adjacent to the BH laser, the wiring parasitic capacity between devices is small, and high speed operation is possible. In addition, since the heterojunction bipolar transistors are formed in the buried layer, the epitaxial growth is required only twice, and the fabrication is easy.
In said conventional optoelectronic integrated circuit, when an attempt is made to raise the operating speed over 1 GHz, since the P type InP buried layer is forming a PN junction nearly over the entire surface of the (heterojunction bipolar transistor) collector layer and an N type InP cladding layer of the BH laser, the junction capacity is large, and when an attempt is made to lower the resistance of the collector layer, the carrier density of the collector layer cannot be raised over 2.times.10.sup.17 cm.sup.-8 in order to set the breakdown voltage over 10 V due to the relationship of the breakdown voltage between the collector layer and separation layer, and the collector resistance is high, and since the carrier density of collector layer is 1.times.10.sup.17 cm.sup.-3, the contact resistance of the collector is as high as 30 .OMEGA., which was an obstacle to increasing the operation speed.
In the prior art, moverover, each layer of a heterojunction bipolar transistor is formed by a second liquid phase epitaxial growth, but since the reverse mesa shape has been already formed at this time, and the surface is not flat, the film thickness of each layer is uneven within the plane. Therefore, the thickness of base layer differs with the heterojunction bipolar transistor device, and the current gain varies with the device. The nonuniformity particularly matters for more sophisticated optoelectronic integrated circuits.
Still more, when an attempt is made to raise the speed of heterojunction bipolar transistor, it is necessary to raise the carrier density of the base, lower the base resistance, lower the carrier density of the collector contacting the base, and reduce the collector capacity, but when an attempt is made to raise the carrier density of the base, the base impurity (for example, Zn) diffuses again into the collector side at the time of the second liquid phase epitaxial growth, and a band gap to block the diffusion of the few carriers not present in the base is formed in a wide layer to lower the current gain.
In the prior art, yet, thermal diffusion is conducted for a long period of time in order to separate the devices electrically, and by this heat treatment, thermal damage was induced to lower the yield of the circuit.